The energy needed to operate a power network is supplied by several different types of power plants. In this context, most power plants such as, for instance, nuclear power plants, coal-burning power plants, natural-gas power plants, wind power plants, biogas power plants or solar power plants are merely energy producers that feed energy into the non-local (or else external) power network. Non-local power networks are, for instance, transmission networks such as those operated in Germany, for example, by Amprion, 50 Hertz, Tennet and TransnetEnBW. These transmission networks are part of the European interconnection grid. As pure energy producers, the above-mentioned power plants cannot absorb surplus energy from the power network and store it if the need arises. Energy storage systems, in contrast, can be employed to absorb energy and release it into a power network. Examples of energy storage systems are centralized energy storage systems such as pumped storage power plants, or else decentralized energy storage systems such as batteries or flywheel energy storage systems. The pumped storage power plants constitute largely weather-independent energy storage systems so that, as a rule, they are always available. Centralized energy storage systems are normally dimensioned for a large capacity. In order to provide an operating reserve for the non-local power network, centralized energy storage systems, owing to the available output, are suitable for such a purpose in the non-local power network. Pumped storage power plants can have an output of several 100 MW or more, depending on their size, although the generators are usually configured to produce electricity at full load and thus can instantaneously utilize the full output of the pumped storage power plant with a corresponding level of efficiency. This mode of operation does not lend itself for stabilizing or improving the local network quality in a power network having a power demand that is quite negligible in comparison to the capacity of the pumped storage power plant.
Centrally employed battery storage systems are currently under construction with the aim of implementing a pilot operation for network-stabilizing (non-location-bound) tasks (operating reserve). The systems planned up until now, however, do not fulfill any location-bound tasks. Fundamentally speaking, however, battery storage systems, owing to their inherent interrelationships between output, capacity and ageing, are not well-suited for such applications involving several load cycles per day, and they degrade quickly due to temperature influences, system failures and faulty operation. For this reason, battery storage systems are very high-maintenance. Moreover, due to their high fire and chemical risks, battery storage systems pose a hazard to the environment and/or to water, thus requiring extensive safeguarding resources.
Decentralized energy storage systems are generally optimized for the stabilization of the local power demand and they are not configured or qualified to supply operating reserve to support the non-local power network. Such systems cannot make a contribution to meeting the demand for all power networks. Up until now, the decentralized storage systems have not been interconnected to form a system that operates locally and non-locally.
U.S. Pat. No. 7,400,052 B1 discloses a transient energy system for a load that, under normal conditions, is fed in via an intermediate DC bus exclusively from the network as the primary power source. If the primary power supply fails, then the load is temporarily supplied with energy via a transient power supply that is likewise connected to the DC bus. In this context, the transient power supply comprises two separate sources of transient power, whereby a flywheel energy storage system is provided as a fast and brief transient power supply in the case of network failure times within the range of 1 second, while a gas turbine with a gas reservoir is provided as the second transient power supply for bridging longer network failures. Here, in case of a network failure, during a first, brief time interval, it is exclusively the flywheel energy storage system that supplies power to the DC bus, whereas during the subsequent second time interval, the flywheel energy storage system as well as the gas turbine power plant feed energy into the DC bus and, during the subsequent third time interval, only the gas turbine power plant feeds energy into the DC bus, while the flywheel energy storage system is recharged from the DC bus. Therefore, the gas turbine power plant also takes over the power supply of the flywheel energy storage system which, under normal conditions, takes place by means of the grid, which is not available during transient operation. In order for such energy storage installations to be able to reliably fulfill their tasks, they have to be equipped with transient aggregates so that they can bridge network failures during the first and second time intervals. In the case of network failures lasting longer, the gas turbine power plant itself can take over the transient power supply as the source of energy and consequently, it has to be dimensioned to be sufficiently large, which is complicated and restricts the mobility of decentralized energy storage units, thus rendering their operation more difficult. In this context, the suppliers, namely, (a) the flywheel energy storage system and (b) the gas turbine power plant, feed energy into the DC bus only in case of a network failure and, in U.S. Pat. No. 7,400,052 B1, they are not employed to supply the operating reserve to support the external power network while the grid is available. Moreover, here, the operating aggregates of the flywheel energy storage systems are not fed from the DC bus and consequently, they need to have a separate power supply.
For this reason, it would be desirable to have access to an effective, environmentally friendly, fail-safe and easy-to-operate energy storage system having a large capacity that, depending on the demand, can feed energy into connected external power networks or else can absorb energy from them and, in the case of a network failure, can nevertheless remain ready for operation.